The present invention relates to preparation of semiconductor-grade silicon.
The large crystals of silicon, from which the wafers used in integrated circuit device fabrication are cut, are grown from a silicon melt, and, while the crystal which is pulled from the melt is normally substantially purer than the melt itself, it is still necessary to have an extremely high-purity starting stock of silicon for the melt, in order to pull crystals having the extremely low impurity concentrations required for integrated circuit device fabrication. The present invention teaches an improved process for providing the silicon starting stock from which crystals are grown.
Normally, metallurgical-grade silicon, such as may be produced by direct reaction of coke and silica in a furnace, is reacted with HCl at 300.degree. C. to form trichlorosilane. This first step of processing leaves behind much of the impurity in metallurgical silicon. The trichlorosilane itself can be filtered and redistilled, to further refine its impurity. The trichlorosilane is then reduced to form elemental silicon of reasonably high purity. It is of course possible to use other silicon compounds for the reaction which deposits silicon, and the present invention is applicable generally to processes in which silicon is deposited from a gas-phase reaction.
Conventionally, the deposition of silicon from the gaseous phase can be accomplished as disclosed, for example, in U.S. Pat. No. 4,213,937, which teaches a fluidized bed reactor for deposition of silicon. An alternative process is the so-called "Siemens" process, which involves hydrogen reduction of chlorosilanes on an electrically heated silicon filament. A third known process of preparation of silicon (the Union Carbide process) yields very finely divided (almost colloidal) silicon, produced by a free-space reaction.
The present invention provides a new and different process for deposition of silicon from a gas-phase reaction. The present invention deposits silicon as a liquid rather than as a solid, i.e. the deposition zone is held above the melting point of silicon (1410.degree. C.). While deposition of silicon above its melting point has been previously described in the published literature (see M. Bawa, "Hydrogen Reduction of Chlorosilanes", Semiconductor Engineering Journal, Vol. 1, No. 3, page 42 (1980)), the present invention teaches at least two features of a liquid-silicon deposition process which are not taught or suggested by the Bawa article. The present invention deposits silicon above its melting point on a bed of silicon nitride particles having a high total surface area. Since silicon nitride is wetted by silicon, the silicon drips down through the silicon nitride particle bed, and can be collected at the bottom of the reactor. The previous technology regarding deposition of silicon above its melting point was not commercially exploited, because of the lack of suitable materials for configuration of such a reactor. However, the present invention teaches using a silicon nitride reactor, which may contain all nitride parts formed by the process taught in simultaneously-filed application Ser. No. 452,484.
Thus it is an object of the present invention to provide a suitable process for deposition of silicon above its melting point.
Deposition of silicon above its melting point is desirable in part because the efficiency of reaction of the process gases is at a maximum very close to the melting point of silicon. Thus, a liquid-deposition process is inherently more efficient than a lower-temperature deposition process. The prior art processes typically achieve deposition at temperatures in the neighborhood of 1200.degree. C.
A very important difficulty with growth of semiconductor-grade crystals when using prior art silicon-deposition processes such as the Siemens process or fluidized-bed technology is that the intermediate stage silicon, i.e. the polycrystalline silicon which is formed by the deposition process, must be handled and exposed to the atmosphere, and can absorb undesirable impurities while it remains in this intermediate stage.
A further object of the invention is therefore to provide a method for growth of crystals of semiconductor-grade silicon, which does not require handling of any intermediate stage of bulk silicon which has a large surface area.
A further difficulty of prior methods for preparing the feedstock for silicon crystal pullers is that they are not inherently well suited to small-scale production processes. Thus, the economies of scale involved in bulk polysilicon production tend to impose a barrier to entry which either precludes competition by smaller entry level enterprises or forces them to be dependent on supplies received from larger corporations.
Thus it is an object of the present invention to provide a method for provision of silicon feedstock to silicon crystal pullers which is inherently suitable for efficient small-scale operation.
A further important innovative feature of the invention is the provision for an intermediate crystal pulling step. That is, in the presently preferred embodiment, the liquid as-deposited silicon is directly transferred to a first crystal puller, from which a first rod of polycrystalline or crystalline silicon is pulled. Since impurity segregation occurs at this stage, the rod which is thus pulled is more pure than the liquid silicon from which it is pulled. In addition, the pulled rod itself has low surface area, and can therefore be handled and stored much more safely than large-surface-area forms of bulk silicon.
In one aspect of the invention, there is provided a means for the deposition of silicon, at a temperature above its melting point, on a large surface area column of silicon nitride. This arrangement permits gravity feed of the liquid silicon to a reservoir below the column. The reservoir is connected to a crystal puller apparatus to provide recharging capability to the crystal pulling operation.
The means for deposition preferably comprises a vessel of high purity silicon nitride, containing particles of silicon nitride. Trichlorosilane and hydrogen is introduced into the vessel containing the heated silicon nitride, where it is subjected to a hydrogen reduction process. This process results in deposition of liquid silicon on the silicon nitride particles. Gravity flow carries the liquid silicon to a reservoir, which is preferably connected with a crystal pulling apparatus. All of the items mentioned are fabricated of high purity silicon nitride, so that all the surfaces that come in contact with the liquid silicon are formed from high purity materials. This removes these items from the list of possible sources of impurities.
In another aspect of the invention, there is provided a method for deposition of silicon on a silicon nitride matrix at a temperature above the melting point of silicon. The method begins with a vessel containing a column of high purity silicon nitride particles enclosed in a vessel of high purity silicon nitride. A reservoir to collect the liquid silicon, and connecting piping to transfer molten silicon from the reservoir to a crystal pulling operation, are also made of high purity silicon nitride. All these parts are heated to ensure the silicon is maintained in a liquid state until a crystal is grown. Trichlorosilane and hydrogen is introduced into the column containing silicon nitride particles, and hydrogen reduction of the trichlorosilane takes place, with silicon being deposited on the large surface area of the silicon nitride particles. The liquid silicon is collected by gravity flow in a reservoir, and a liquid transfer system carries the silicon to the melt reservoir of a crystal pulling apparatus. A crystal rod is grown from the liquid, the rod having a very high purity.
According to the present invention there is provided: a process for producing silicon from a silicon bearing gas flow, comprising the steps of: providing a matrix of silicon nitride particles; forcing through a portion of said matrix of silicon nitride particles a stream of a silicon bearing gas mixture; said matrix of silicon nitride particles being heated to above the melting temperature of silicon while said gas flow is passed through said matrix; and collecting at the bottom of said matrix of silicon nitride particles liquid silicon deposited of said nitride particles from said gas stream.
A freezing valve for liquid silicon, in which an S trap is used to facilitate obstruction of the passageway by the frozen silicon, is disclosed in a report submitted by Energy Materials Corporation, of Harvard, Mass., to the U.S. Department of Energy under JPL Contract 955269, "Gaseous Melt Replenishment System". The authors of this report are D. Jewett et al. A trap configuration for the freezing valve, such as disclosed in this publicly available report, is preferably used in practicing the present invention.
Liquid transfer of silicon, using quartz plumbing, is believed to have been exemplified in products marketed by Siltec Company for several years now.